The present invention relates to a mass spectrometer and an ion source used therefor and, in particular, it relates to an ion source constitution suitable to ionization of compounds present in solutions under an atmospheric pressure and introduction of resultant ions of the compounds into a mass spectrometer exhausted to a high vacuum, and a mass spectrometer using the ion source.
While compounds present in the solutions can be separated by a capillary electrophoresis system (CE) or a liquid chromatograph (LC), it is difficult to identify the composition of the separated compound. On the other hand, while the composition of the compounds can be identified at a high sensitivity by a mass spectrometer (MS), the compounds in the solution can not be separated. Thus, when a plurality of compounds dissolved in a solvent such as water are separated and analyzed, a capillary electrophoresis/mass spectrometer (CE/MS) comprising a combination of a mass spectrometer and a capillary electrophoresis system or a liquid chromatograph/mass spectrometer (LC/MS) comprising a combination of the mass spectrometer and a liquid chromatograph have been used generally. For analyzing the compound separated by the capillary electrophoresis system or the liquid chromatograph by the mass spectrometer, it is necessary to transform the compound molecules in the solution into gaseous ions. As a method of obtaining such gaseous ions, for example, an ion spray method (refer to Analytical Chemistry, vol. 59 (1987), pp. 2642-2646) has been known in the prior art. In the ion spray method, a specimen is introduced from one end of a fused-silica capillary (hereinafter referred to as a capillary) and nebulized from the other end (top end) into atmospheric pressure. A gas is supplied along the axial direction for the outer circumference of the capillary. A high voltage (3-6 kV) is applied between the capillary and a sampling orifice for passing the ions of the compounds in the solution into the mass spectrometer exhausted to a high vacuum, and a strong electric field is formed at the top end of the capillary. By the strong electric field, the specimen solution nebulized from the top end of the capillary is transformed into fine charged droplets by a so-called electrospraying phenomenon. Further, the ingredients of the solvent in the charged droplets are evaporated by a gas flow supplied along the outer circumference of the capillary to form gaseous ions of the compound. The formed ions of the compound are introduced by way of the sampling orifice into the mass spectrometer and mass spectralyzed. The gas flow promotes evaporation of the charged droplets, as well as suppresses the occurrence of electric discharge at the top end of the capillary.
Further, a method has also been proposed for ionizing compounds by setting the flow rate of the solution of the compounds supplied in the capillary to not more than 10 .mu.l/min without supplying the gas flow to the outer circumference of the capillary (Journal of Physical Chemistry, vol. 88 (1984), pp. 4451-4459). Although the method is distinguished from the ion spray method described above, it is in common with that of the ion spray method regarding the principle of forming the ions. Further, an atmospheric pressure chemical ionization method (Analytical Chemistry, vol. 54 (1982), pp. 143-146) adopts a method of disposing an electrode near the top end of the capillary for generating electric discharge and ionizing droplets nebulized under an atmospheric pressure by the discharge.
In each of the spray ionizing methods described above, it is considered necessary to form fine charged droplets having a diameter of about not more than 10 nm in order to attain a high ion forming efficiency. U.S. Pat. No. 5,352,892 discloses a method of disposing a liquid shield before a mass spectrometer for allowing only a central portion of a spray in which a number of droplets of small particle diameter are gathered to pass for ionizing the ingredients of the specimen more efficiently by the ion spray method. Further, Japanese Patent Laid-Open Sho 61-194349 describes a method of disposing a sheet punctured at a center before a mass spectrometer to suppress spreading of the spray and to introduce only a portion of the spray with a small spreading angle into the mass spectrometer in order to attain efficient ionization by the atmospheric pressure chemical ionization method. Further, Japanese Patent Laid-Open Hei 07-159377 describes a method of disposing means for mixing sprays in order to promote the solvent evaporation effect for efficient ionization also in the atmospheric pressure chemical ionization method. Further, Japanese Patent Laid-Open Hei 5-256837 describes a method of heating the droplets formed by the nebulizing to promote evaporation.
Recently, an ionization method referred to as a sonic spray method has been reported which can efficiently form ions by merely nebulizing a sample solution with a gas at a sonic velocity (refer to Analytical Chemistry, vol. 66, (1994), pp. 4557-4559; Analytical Chemistry, vol. 67, (1995), pp. 2878-2882 and Japanese Patent Laid-Open Hei 07-306193). It is considered that fine charged droplets are formed by the flow of a gas at a sonic velocity and molecules of the solvent are evaporated to form ions in this method.
In the sonic spray method described above, a gas stream at a high speed is caused to flow in a close vicinity of a sampling orifice of a mass spectrometer. Accordingly, if the flow rate of the specimen solution is increased, a great amount of charged droplets are formed and passed from the orifice into the inside of the mass spectrometer, and charged droplets cooled by adiabatic expansion are clustered into large droplets to cause a problem of due in great noises which result. Accordingly, this method can not be used in a case where the flow rate of the specimen solution is as high as about 1 ml/min. Further, it also requires an operation for adjusting the gas flow rate in accordance with the flow rate of the specimen solution or the kind of the solvent, thereby inposing a large burden on users.